Cell, in biology, the basic unit of all living matter. All forms of life consist of cells or groups of cells. Some, such as bacteria, consist of single cells, while others, such as redwood trees and whales, contain billions of cells. (Viruses, which some scientists consider to be living things, are not made up of cells.)

Most cells are so tiny that they can be seen only through a microscope. A few kinds of cells, however, are large enough to be seen with the unaided eye. The giant amoeba, for example, is a single-celled organism that can stretch to as long as one-fifth of an inch (5 mm) when it is moving actively. Cells in complex multicellular organisms form thousands of different shapes. The shape of a cell and the internal structures of a cell are related to its particular function.

The shape of a cellThe shape of a cell is a result of its particular function.

Robert Hooke, in 1665, was the first to identify cells; he also gave them their name. In the early 19th century, Matthias Schleiden, Theodor Schwann, and others recognized the universal occurrence and importance of cells. Cell biology, one of the most important branches of biological science, includes a number of disciplines, including biochemistry, genetics, and cytology (the study of structure and function of the parts of the cell). Research on the causes of cancer and many other diseases has become a major concern of cell biology, as has research on development of embryos.